Beginning the act of substance abuse. That first sip or that first hit signals the push off from the hypothetical shore we call reality. The more fucked up you get the further out you are drifting. One doesn't know how far the current will take them out this evening or if they will even return.

When you eat dirt for the first time and leave your family to build a shack in the woods

The act of "pushing" the plunger of a syringe. See also: shooting up.